Blender class register issue
The bl_idname of MESH_OT_L should be lower case and one dot only, which is: mesh.l.

You need to follow the naming convention that Blender API required. It is kind of meta type that Blender used to organize the components that addon gonna registered.
And also you should use MESH_OT_L.bl_idname in your panel for clearance and flexible to change your bl_idname since it will warn you if MESH_OT_L is not found or missing bl_idname.
It is surely included in the page that you point out: Reference/Release Notes/2.80/Python API/Addons - Blender Developer Wiki
But it is not that clear why should lower case id been used (it is a former convention rather than 2.80 change)